In situ observation of thermal-driven degradation and safety concerns of lithiated graphite anode

Graphite, a robust host for reversible lithium storage, enabled the first commercially viable lithium-ion batteries. However, the thermal degradation pathway and the safety hazards of lithiated graphite remain elusive. Here, solid-electrolyte interphase (SEI) decomposition, lithium leaching, and gas release of the lithiated graphite anode during heating were examined by in situ synchrotron X-ray techniques and in situ mass spectroscopy. The source of flammable gas such as H2 was identified and quantitively analyzed. Also, the existence of highly reactive residual lithium on the graphite surface was identified at high temperatures. Our results emphasized the critical role of the SEI in anode thermal stability and uncovered the potential safety hazards of the flammable gases and leached lithium. The anode thermal degradation mechanism revealed in the present work will stimulate more efforts in the rational design of anodes to enable safe energy storage.

As shown, without lithium intercalation, the graphite showed no phase transformation when heated to 280°C. The graphite 2H (002), (100), (101), and (102) Bragg reflection only showed slight shifts to lower angles due to thermally induced lattice expansion. However, no phase transformation can be observed. The results confirmed that the unlithiated graphite is stable against the PVDF and carbon additive even at elevated temperature. However, with lithium intercalation in the graphite anode, the intercalated lithium is the reason for the instability of the lithiated graphite anode, which was gradually leached out of the graphite layer during the heating process.  To calculate the amount of lithium consumption by PVDF, assuming 0.2 g PVDF with 9. The entropy change of the above reaction based on the 1 mole oxygen is -1197.46 kJ/mol.

Anode phase ratio
In our oxygen exposure reaction, the amount of lithium is limited. Thus, based on lithium, the reaction entropy is -299.4 kJ/molLi (-1197.46/4), which is also -299.4/6.95 =-43.14 kJ/gLi. Therefore, the theoretical value of heat generation by inducing oxygen at different temperature can be calculated by multiply -43.14 kJ/gLi and the amount of leached lithium at that temperature.

Supplementary Method
Materials In situ high-energy XRD during heating with mass spectrometry. A beamline standard timeresolved high-energy X-ray diffraction (HEXRD) system 1 with a residual gas analyzer was utilized at Beamline 17 BM of Advanced Photon Source (APS) 2 to characterize the phase transformation during heating, with the X-ray wavelength of 0.24105Å. The lithiated anode powder (~2 mg) was loaded into a quartz capillary tube (7 mm) with a controllable heating unit and helium protective gas flow (Supplementary Figure 2). Ultrahigh-purity helium was used as the carrier gas (flow at 5 ml/min), and a residual gas analyzer (Pfeiffer Vacuum PrismaPlus ® QMG 220) was utilized to measure the outgassing of the anode during heating. The heating rate was 2°C/min from room temperature to 280°C; the HEXRD data acquisition rate was 30 seconds per image by the 2D detector. The 2D HEXRD data were then converted and analyzed with GSAS II software.
The  Figure 4a in the main text, the material was collected from the graphite anode laminate before charge. And for Figure 4c in the main text, the material was collected after the in situ HEXRD measurement with natural cooling to room temperature after reached 280°C.
In situ pair distribution function measurement and analysis. Synchrotron X-ray total scattering data were collected on beamline 11-ID-C at the Advanced Photo Source (APS), ANL. The rapidacquisition PDF method was used with a wavelength of λ = 0.1173 Å. 5 A PerkinElmer amorphous Si two-dimensional image-plate detector (2048 × 2048 pixels and 200 × 200 m pixel size) was used at a distance of ~400 mm. The two-dimensional data were converted to one-dimensional XRD data using the GSAS-II software. 6 PDF data were obtained from Fourier transformation of the background and Compton scattering corrected data S(Q) in xPDFsuite software over a Q range of 0.4-19 Å-1. 7,8 A Linkam THM600 furnace with temperature control <0.1 o C was used to heat the lithiated powder, which was sealed in Kapton tape, see the setup as Supplementary Figure 15a.
The heating rate was set at 5 o C/min, then the sample was hold at each target temperature for at least 10 min before each PDF data acquisition. The PDF acquisition time was 20 min per image, and for each image a CeO2 standard powder (CeO2 NIST 647b) calibration was carried out. The furnace was calibrated between 30 o C to 350 o C by using a MgO lattice parameter as standard 9 before the experiment, see Supplementary Figure 15b.